Test for feature/arch assumptions

[workingjubilee]

We assume:

• x86_64 targets have SSE, SSE2 by default <--- this is wrong, probably, as Knight's Landing has no xmm registers so it apparently doesn't support SSE! However, this is an extremely niche supercomputer processor.
• aarch64 targets have Neon by default

We should test these assumptions, and document any other assumptions we accumulate as we go along.